LLVM 2.9 Release Notes

This document contains the release notes for the LLVM Compiler
Infrastructure, release 2.9. Here we describe the status of LLVM, including
major improvements from the previous release and significant known problems.
All LLVM releases may be downloaded from the LLVM releases web site.

Note that if you are reading this file from a Subversion checkout or the
main LLVM web page, this document applies to the next release, not the
current one. To see the release notes for a specific release, please see the
releases page.

The LLVM 2.9 distribution currently consists of code from the core LLVM
repository (which roughly includes the LLVM optimizers, code generators
and supporting tools), the Clang repository and the llvm-gcc repository. In
addition to this code, the LLVM Project includes other sub-projects that are in
development. Here we include updates on these subprojects.

Clang is an LLVM front end for the C,
C++, and Objective-C languages. Clang aims to provide a better user experience
through expressive diagnostics, a high level of conformance to language
standards, fast compilation, and low memory use. Like LLVM, Clang provides a
modular, library-based architecture that makes it suitable for creating or
integrating with other development tools. Clang is considered a
production-quality compiler for C, Objective-C, C++ and Objective-C++ on x86
(32- and 64-bit), and for darwin/arm targets.

In the LLVM 2.9 time-frame, the Clang team has made many improvements in C,
C++ and Objective-C support. C++ support is now generally rock solid, has
been exercised on a broad variety of code, and has several new C++'0x features
implemented (such as rvalue references and variadic templates). LLVM 2.9 has
also brought in a large range of bug fixes and minor features (e.g. __label__
support), and is much more compatible with the Linux Kernel.

If Clang rejects your code but another compiler accepts it, please take a
look at the language
compatibility guide to make sure this is not intentional or a known issue.

DragonEgg is a
gcc plugin that replaces GCC's
optimizers and code generators with LLVM's.
Currently it requires a patched version of gcc-4.5.
The plugin can target the x86-32 and x86-64 processor families and has been
used successfully on the Darwin, FreeBSD and Linux platforms.
The Ada, C, C++ and Fortran languages work well.
The plugin is capable of compiling plenty of Obj-C, Obj-C++ and Java but it is
not known whether the compiled code actually works or not!

The 2.9 release has the following notable changes:

The plugin is much more stable when compiling Fortran.

Inline assembly where an asm output is tied to an input of a different size
is now supported in many more cases.

Basic support for the __float128 type was added. It is now possible to
generate LLVM IR from programs using __float128 but code generation does not
work yet.

The new LLVM compiler-rt project
is a simple library that provides an implementation of the low-level
target-specific hooks required by code generation and other runtime components.
For example, when compiling for a 32-bit target, converting a double to a 64-bit
unsigned integer is compiled into a runtime call to the "__fixunsdfdi"
function. The compiler-rt library provides highly optimized implementations of
this and other low-level routines (some are 3x faster than the equivalent
libgcc routines).

In the LLVM 2.9 timeframe, compiler_rt has had several minor changes for
better ARM support, and a fairly major license change. All of the code in the
compiler-rt project is now dual
licensed under MIT and UIUC license, which allows you to use compiler-rt
in applications without the binary copyright reproduction clause. If you
prefer the LLVM/UIUC license, you are free to continue using it under that
license as well.

LLDB is a brand new member of the LLVM
umbrella of projects. LLDB is a next generation, high-performance debugger. It
is built as a set of reusable components which highly leverage existing
libraries in the larger LLVM Project, such as the Clang expression parser, the
LLVM disassembler and the LLVM JIT.

libc++ is another new member of the LLVM
family. It is an implementation of the C++ standard library, written from the
ground up to specifically target the forthcoming C++'0X standard and focus on
delivering great performance.

In the LLVM 2.9 timeframe, libc++ has had numerous bugs fixed, and is now being
co-developed with Clang's C++'0x mode.

Like compiler_rt, libc++ is now dual
licensed under the MIT and UIUC license, allowing it to be used more
permissively.

LLBrowse is an interactive viewer for LLVM modules. It can load any LLVM
module and displays its contents as an expandable tree view, facilitating an
easy way to inspect types, functions, global variables, or metadata nodes. It
is fully cross-platform, being based on the popular wxWidgets GUI toolkit.

The VMKit project is an implementation
of a Java Virtual Machine (Java VM or JVM) that uses LLVM for static and
just-in-time compilation. As of LLVM 2.9, VMKit now supports generational
garbage collectors. The garbage collectors are provided by the MMTk framework,
and VMKit can be configured to use one of the numerous implemented collectors
of MMTk.

An exciting aspect of LLVM is that it is used as an enabling technology for
a lot of other language and tools projects. This section lists some of the
projects that have already been updated to work with LLVM 2.9.

Crack Programming Language

Crack aims to provide the
ease of development of a scripting language with the performance of a compiled
language. The language derives concepts from C++, Java and Python, incorporating
object-oriented programming, operator overloading and strong typing.

TTA-based Codesign Environment (TCE)

TCE is a toolset for designing application-specific processors (ASP) based on
the Transport triggered architecture (TTA). The toolset provides a complete
co-design flow from C/C++ programs down to synthesizable VHDL and parallel
program binaries. Processor customization points include the register files,
function units, supported operations, and the interconnection network.

TCE uses Clang and LLVM for C/C++ language support, target independent
optimizations and also for parts of code generation. It generates new LLVM-based
code generators "on the fly" for the designed TTA processors and loads them in
to the compiler backend as runtime libraries to avoid per-target recompilation
of larger parts of the compiler chain.

PinaVM

PinaVM is an open
source, SystemC front-end. Unlike many
other front-ends, PinaVM actually executes the elaboration of the
program analyzed using LLVM's JIT infrastructure. It later enriches the
bitcode with SystemC-specific information.

Pure

Pure is an
algebraic/functional
programming language based on term rewriting. Programs are collections
of equations which are used to evaluate expressions in a symbolic
fashion. The interpreter uses LLVM as a backend to JIT-compile Pure
programs to fast native code. Pure offers dynamic typing, eager and lazy
evaluation, lexical closures, a hygienic macro system (also based on
term rewriting), built-in list and matrix support (including list and
matrix comprehensions) and an easy-to-use interface to C and other
programming languages (including the ability to load LLVM bitcode
modules, and inline C, C++, Fortran and Faust code in Pure programs if
the corresponding LLVM-enabled compilers are installed).

Pure version 0.47 has been tested and is known to work with LLVM 2.9
(and continues to work with older LLVM releases >= 2.5).

IcedTea Java Virtual Machine Implementation

IcedTea provides a
harness to build OpenJDK using only free software build tools and to provide
replacements for the not-yet free parts of OpenJDK. One of the extensions that
IcedTea provides is a new JIT compiler named Shark which uses LLVM
to provide native code generation without introducing processor-dependent
code.

OpenJDK 7 b112, IcedTea6 1.9 and IcedTea7 1.13 and later have been tested
and are known to work with LLVM 2.9 (and continue to work with older LLVM
releases >= 2.6 as well).

Glasgow Haskell Compiler (GHC)

GHC is an open source, state-of-the-art programming suite for Haskell,
a standard lazy functional programming language. It includes an
optimizing static compiler generating good code for a variety of
platforms, together with an interactive system for convenient, quick
development.

Polly - Polyhedral optimizations for LLVM

Polly is a project that aims to provide advanced memory access optimizations
to better take advantage of SIMD units, cache hierarchies, multiple cores or
even vector accelerators for LLVM. Built around an abstract mathematical
description based on Z-polyhedra, it provides the infrastructure to develop
advanced optimizations in LLVM and to connect complex external optimizers. In
its first year of existence Polly already provides an exact value-based
dependency analysis as well as basic SIMD and OpenMP code generation support.
Furthermore, Polly can use PoCC(Pluto) an advanced optimizer for data-locality
and parallelism.

Rubinius

Rubinius is an environment
for running Ruby code which strives to write as much of the implementation in
Ruby as possible. Combined with a bytecode interpreting VM, it uses LLVM to
optimize and compile ruby code down to machine code. Techniques such as type
feedback, method inlining, and deoptimization are all used to remove dynamism
from ruby execution and increase performance.

FAUST is a compiled language for real-time
audio signal processing. The name FAUST stands for Functional AUdio STream. Its
programming model combines two approaches: functional programming and block
diagram composition. In addition with the C, C++, JAVA output formats, the
Faust compiler can now generate LLVM bitcode, and works with LLVM 2.7-2.9.

Type Based Alias Analysis (TBAA) is now implemented and turned on by default
in Clang. This allows substantially better load/store optimization in some
cases. TBAA can be disabled by passing -fno-strict-aliasing.

This release has seen a continued focus on quality of debug information.
LLVM now generates much higher fidelity debug information, particularly when
debugging optimized code.

Inline assembly now supports multiple alternative constraints.

A new backend for the NVIDIA PTX virtual ISA (used to target its GPUs) is
under rapid development. It is not generally useful in 2.9, but is making
rapid progress.

LLVM IR now supports the unnamed_addr
attribute to indicate that constant global variables with identical
initializers can be merged. This fixed an
issue where LLVM would incorrectly merge two globals which were supposed
to have distinct addresses.

In addition to a large array of minor performance tweaks and bug fixes, this
release includes a few major enhancements and additions to the optimizers:

Link Time Optimization (LTO) has been improved to use MC for parsing inline
assembly and now can build large programs like Firefox 4 on both Mac OS X and
Linux.

The new -loop-idiom pass recognizes memset/memcpy loops (and memset_pattern
on darwin), turning them into library calls, which are typically better
optimized than inline code. If you are building a libc and notice that your
memcpy and memset functions are compiled into infinite recursion, please build
with -ffreestanding or -fno-builtin to disable this pass.

A new -early-cse pass does a fast pass over functions to fold constants,
simplify expressions, perform simple dead store elimination, and perform
common subexpression elimination. It does a good job at catching some of the
trivial redundancies that exist in unoptimized code, making later passes more
effective.

A new -loop-instsimplify pass is used to clean up loop bodies in the loop
optimizer.

LLVM has a new RegionPass
infrastructure for region-based optimizations.

Several optimizer passes have been substantially sped up:
GVN is much faster on functions with deep dominator trees and lots of basic
blocks. The dominator tree and dominance frontier passes are much faster to
compute, and preserved by more passes (so they are computed less often). The
-scalar-repl pass is also much faster and doesn't use DominanceFrontier.

The Dead Store Elimination pass is more aggressive optimizing stores of
different types: e.g. a large store following a small one to the same address.
The MemCpyOptimizer pass handles several new forms of memcpy elimination.

LLVM now optimizes various idioms for overflow detection into check of the
flag register on various CPUs. For example, we now compile:

The LLVM Machine Code (aka MC) subsystem was created to solve a number
of problems in the realm of assembly, disassembly, object file format handling,
and a number of other related areas that CPU instruction-set level tools work
in.

ELF MC support has matured enough for the integrated assembler to be turned
on by default in Clang on X86-32 and X86-64 ELF systems.

MC supports and CodeGen uses the .file and .loc directives
for producing line number debug info. This produces more compact line
tables and easier to read .s files.

MC supports the .cfi_* directives for producing DWARF
frame information, but it is still not used by CodeGen by default.

The MC assembler now generates much better diagnostics for common errors,
is much faster at matching instructions, is much more bug-compatible with
the GAS assembler, and is now generally useful for a broad range of X86
assembly.

LLVM now has an experimental format-independent object file manipulation
library (lib/Object). It supports both PE/COFF and ELF. The llvm-nm tool has
been extended to work with native object files, and the new llvm-objdump tool
supports disassembly of object files (but no relocations are displayed yet).

Win32 PE-COFF support in the MC assembler has made a lot of progress in the
2.9 timeframe, but is still not generally useful.

We have put a significant amount of work into the code generator
infrastructure, which allows us to implement more aggressive algorithms and make
it run faster:

The pre-register-allocation (preRA) instruction scheduler models register
pressure much more accurately in some cases. This allows the adoption of more
aggressive scheduling heuristics without causing spills to be generated.

LiveDebugVariables is a new pass that keeps track of debugging information
for user variables that are promoted to registers in optimized builds.

The scheduler now models operand latency and pipeline forwarding.

A major register allocator infrastructure rewrite is underway. It is not on
by default for 2.9 and you are not advised to use it, but it has made
substantial progress in the 2.9 timeframe:

A new -regalloc=basic "basic" register allocator can be used as a simple
fallback when debugging. It uses the new infrastructure.

New infrastructure is in place for live range splitting. "SplitKit" can
break a live interval into smaller pieces while preserving SSA form, and
SpillPlacement can help find the best split points. This is a work in
progress so the API is changing quickly.

The inline spiller has learned to clean up after live range splitting. It
can hoist spills out of loops, and it can eliminate redundant spills.

Rematerialization works with live range splitting.

The new "greedy" register allocator using live range splitting. This will
be the default register allocator in the next LLVM release, but it is not
turned on by default in 2.9.

LLVM 2.9 includes a complete reimplementation of the MMX instruction set.
The reimplementation uses a new LLVM IR x86_mmx type to ensure that MMX operations
are only generated from source that uses MMX builtin operations. With
this, random types like <2 x i32> are not turned into MMX operations
(which can be catastrophic without proper "emms" insertion). Because the X86
code generator always generates reliable code, the -disable-mmx flag is now
removed.

The following components of this LLVM release are either untested, known to
be broken or unreliable, or are in early development. These components should
not be relied on, and bugs should not be filed against them, but they may be
useful to some people. In particular, if you would like to work on one of these
components, please contact us on the LLVMdev list.

llvm-gcc is generally very stable for the C family of languages. The only
major language feature of GCC not supported by llvm-gcc is the
__builtin_apply family of builtins. However, some extensions
are only supported on some targets. For example, trampolines are only
supported on some targets (these are used when you take the address of a
nested function).

Fortran support generally works, but there are still several unresolved bugs
in Bugzilla. Please see the
tools/gfortran component for details. Note that llvm-gcc is missing major
Fortran performance work in the frontend and library that went into GCC after
4.2. If you are interested in Fortran, we recommend that you consider using
dragonegg instead.

The llvm-gcc 4.2 Ada compiler has basic functionality, but is no longer being
actively maintained. If you are interested in Ada, we recommend that you
consider using dragonegg instead.

A wide variety of additional information is available on the LLVM web page, in particular in the documentation section. The web page also
contains versions of the API documentation which is up-to-date with the
Subversion version of the source code.
You can access versions of these documents specific to this release by going
into the "llvm/doc/" directory in the LLVM tree.

If you have any questions or comments about LLVM, please feel free to contact
us via the mailing
lists.